NEW YORK • Moderna Inc’s Covid-19 vaccine produced antibodies to the coronavirus in all patients tested in an initial safety trial, federal researchers said, clearing an important milestone as the US continues to grapple with a surge in new infections.

The US biotech firm’s breakthrough is one of the most promising developments yet in the race to develop a vaccine against the virus that has paralysed the world’s economy. The stock jumped in premarket trading, poised to reach a record high, and the optimism lifted global equity markets. Yet, the road to a successful shot is filled with hurdles, and some patients in the trial experienced severe side effects.

“The good news is that this vaccine-induced antibodies,” National Institute of Allergy and Infectious Diseases (NIAID) director Anthony Fauci said. “Not just any kind of antibodies, but neutralising antibodies.”

Fauci, in a telephone interview, called the Moderna data “really quite promising”. Though he said the side effects seen were not alarming and were typical of those experienced with other vaccines, some experts sounded a note of caution.

More than half of participants who got the middle of three doses administered in the trial suffered mild to moderate fatigue, chills, headache and muscle pain. Also, 40% of people in the middle-dose group experienced a fever after the second vaccination. Three of 14 patients given the highest dose experienced severe side effects, but that dose is not being used in larger trials.

“Man, that is a lot of adverse events,” said Tony Moody, a doctor and researcher at the Duke Human Vaccine Institute. He said it would be “unusual” for a vaccine to have this rate of side effects.

On the plus side, Moody said that the antibody levels produced were “really encouraging”. The neutralising antibody levels in the trial produced were equivalent to the upper half of what is seen in patients who are infected with the virus and recover, according to the results published on Tuesday in the New England Journal of Medicine.

Moderna’s shares gained about 15% in the US premarket trading yesterday, as stocks of other companies researching vaccines for the virus also rose. AstraZeneca plc climbed as much as 4.2% in London after ITV.com reported that The Lancet medical journal is due to release positive news on the vaccine the company is developing with University of Oxford researchers.

Moderna’s stock has almost quadrupled in value this year on hopes that the company’s vaccine will gain rapid approval. The vaccine will move into a much larger late-stage trial later this month that is likely to determine whether it is fit for commercial use.

Although stimulating production of neutralising antibodies does not prove a vaccine will be effective, it is considered an important early step in testing. The side effects reported were not severe enough in the majority of patients to preclude further testing, according to the report by the NIAID’s researchers.

The vaccine news came as the pandemic continued thriving throughout the US as cases nationwide increased on Tuesday to 3.4 million, according to data collected by Johns Hopkins University and Bloomberg News. More than 136,117 Americans have died. — Bloomberg